1. Field of Invention
This invention relates to power electronic inverters, and more particularly to methods and apparatus to reduce hotspot temperatures and power losses for power devices in a power electronics inverter for a permanent magnet synchronous motor (PMSM).
2. Background Art
Electric machines, such as an electric or hybrid electric vehicles, can employ electrical energy for propulsion via an electric drive system that can include a power circuit, such as a power electronics inverter, coupled to a motor. In this arrangement, the power circuit can controllably transfer power from a power source to the motor to drive a load, such as the vehicle transaxle. For a three-phase synchronous motor, the power circuit can include an inverter with three phase legs, each leg comprising switches that can be individually controlled to provide a desired inverter output. As an example, drive signals that are a function of a motor's torque requirement can be provided to the inverter by an inverter controller. As is commonly practice, an inverter can be configured for bi-directional current flow so that current can flow from the inverter to the motor as well as from the motor to the inverter.
Because inverter current is tied to motor rotation and torque requirements, problems can arise during a “rotor-lock” or “motor-lock” mode when the angular rotation of the motor is substantially reduced or completely stopped. When a drive torque balances a vehicle weight, for example when a vehicle is climbing a slope, or when an obstacle is blocking a vehicle's wheels, an electric motor's rotation can slow or stop. A controller can attempt to increase torque/rotation by increasing the current provided to the motor. However, because the motor is not rotating sufficiently, the increased current can be concentrated in a single phase leg. For example, the current in one inverter phase leg can be twice the current in the remaining two phase legs. The high current concentration can heat up a switching device in the phase leg and increase the power losses associated with the device operation.
Given this phenomena, switching devices for an inverter are designed to tolerate a particular “hotspot” temperature that can be expected at the device during rotor-lock operation. The greater the hotspot temperature to be tolerated, the larger the size requirements of the device, and ultimately the greater the device cost. Unfortunately, the greater the rotor-lock current concentration and hotspot temperature, the higher the device losses, and the higher the overall operational losses of the power conversion system.
Various attempts have been made to reduce power losses under rotor-lock conditions. A traditional solution is to decrease the pulse-width modulation (PWM) switching frequency to reduce inverter power losses. Although this solution may reduce inverter switching losses, it has limited effects because conduction losses are typically the dominating factor in power device losses. More recently, various software-oriented solutions have been proposed. For example, U.S. patent publication 2010/0185350 entitled “Control Device for Electric-Powered Vehicle and Electric-Powered Vehicle with Control Device as Well as Control Method for Electric-Powered Vehicle, and Computer Readable Recording Medium Bearing Program for Causing Computer to Execute Control Method” and assigned to Toyota Jidosha Kabushiki Kaisha discloses a carrier frequency setting unit that sets a carrier frequency (FC) according to the torque command (TR) of a motor generator and the number of motor rotations (MRN). A PWM signal generation unit generates phase modulation waves corresponding to respective phase voltage commands (Vu,Vv, Vw) and generates phase PWM signals (Pu, Pv, Pw) according to the magnitude relationship between each of the phase modulation waves and a carrier wave having the carrier frequency (FC). A PWM center control unit, when the carrier frequency (FC) is lower than a predetermined frequency, generates a PWM center correction value (ΔCE) for variably controlling a PWM center and outputs to the PWM signal. The Toyota publication teaches a carrier frequency dependent solution to the problem of inverter overheating. However, there remains a need for an economical, hardware-oriented solution that can be employed regardless of, and independent of PWM carrier frequency to reduce device power losses and hotspot temperatures.